Transko特兰斯科品牌VCXO应用说明

Absolute Pull Range vs. Total Pull
Introduction
There is often confusion among engineers when comparing pullability specifications among different manufacturers’ datasheets. Some manufacturers use the absolute pull range (APR) method while others use total pullability method to specify the amount of voltage control adjustment. The purpose of this application note is to clarify the differences between the two methods.
Definition
In order to define APR, a basic understanding of a VCXO is required. A VCXO is an oscillator that can vary the output frequency based on an input control voltage. The amount of frequency variation,specified in ppm, is the pullability. Manufacturers often use the terms pull, pullability, frequency modulation and frequency deviation interchangeably. For the purpose of this article, we will refer to pullability as total pullability as it is the raw measurement of the frequency variation. This is the
more common method of specifying pullability in a VCXO. However, some manufacturers often use the APR method instead of the total pullability method.The APR method is defined as the amount of frequency variation after accounting for errors due to
variations in temperature, load, supply voltage, and aging.
APR = Total Pullability – (errors due to variations in temperature, load, supply voltage, and aging)
As you can see, the total pullability must be greater than the APR value in order to account for the errors. This is often where the confusion resides. Engineers often compare APR values from one manufacturer against total pullability from another manufacturer as the same spec and assume that one has much more pull than the other. However, this is often not the case. 
Example
Let’s take an example of a VCXO used in a PLL that needs to track a ±50ppm reference. Below is a typical specification for two VCXO. VCXO A uses the total pullability method while VCXO B uses APR method.

At first glance, VCXO A appears to have more pullability, but VCXO B actually has more pullability when all errors are taken into account. If the application requires the PLL to track ±50ppm, the VCXO A may not be suitable.
In effect, the APR value can be considered the “usable pullability”. Instead of calculating the errors, the user can simply specify the APR that meets the PLL tracking requirement and be done. This is often the case in Stratum4 applications where a maximum error of ±32ppm is allowed. Rather than calculating the pullability, specifying APR value of ±32ppm will allow the clock to maintain lock under all environmental conditions. 

Transko特兰斯科品牌VCXO应用说明

绝对拉力范围与总拉力

介绍
在比较不同制造商的数据表中的可牵引性规格时，工程师们经常会感到困惑。一些制造商使用绝对牵引范围（APR）方法，而另一些制造商使用总牵引能力方法来指定电压控制调整量。本申请说明的目的是澄清这两种方法之间的差异。

释义
为了定义APR，需要对VCXO有基本的了解。VCXO晶振是一种可以根据输入控制电压改变输出频率的振荡器。频率变化量，以ppm为单位，是可拉出性。制造商经常使用“拉力”、“可拉动性”、“频率”等术语
调制和频率偏移可互换。出于本文的目的，我们将可拉性称为总可拉性，因为它是频率变化的原始测量。这是在VCXO贴片晶振中指定可拉性的更常见方法

然而，一些制造商经常使用APR方法，而不是总可拉性方法。APR方法被定义为在考虑了由于温度、负载、电源电压和老化变化引起的误差后的频率变化量

APR=总牵引力-（由于温度、负载、电源电压和老化变化而产生的误差）

正如你所看到的，总可拉性必须大于APR值，才能解释错误。这往往是困惑所在。工程师经常将一个制造商的APR值与另一个制造商相同规格的总拉力进行比较，并认为其中一个制造商比另一个具有更大的拉力。然而，情况往往并非如此。

实例
让我们以PLL中使用的VCXO石英晶振为例，该PLL需要跟踪±50ppm参考。以下是两个VCXO的典型规格。VCXO A使用总可拉性方法，而VCXO B使用APR方法。

*需要注意的是，上面的稳定性误差是最大值，所代表的情况是最坏的情况。以下是使用等效规格的两个VCXO的比较。

*需要注意的是，上面的稳定性误差是最大值，所代表的情况是最坏的情况。
以下是使用等效规格的两个VCXO压控晶体振荡器的比较。

乍一看，VCXO A似乎具有更强的可拉性，但当考虑到所有错误时，VCXOB实际上具有更强的拉性。如果应用程序要求PLL跟踪±50ppm，则VCXO A可能不合适。

实际上，APR值可以被认为是“可用的可拉性”。用户可以简单地指定满足PLL跟踪要求的APR，而不是计算误差。Stratum4应用中经常出现这种情况，允许最大误差为±32ppm。指定APR值为±32ppm将允许时钟在所有环境条件下保持锁定，而不是计算可拉出性。